This invention relates generally to a method of cold on-column injection of a liquid sample onto a capillary gas chromatography column and more particularly to such a method that can be used with a relatively larger sample size without producing the peak distortion or splitting observed under conventional on-column injection conditions.
The two principal objectives of a sampling technique in capillary gas chromatography are to allow identical composition for sample injected onto the column and sample prior to the injection, and to introduce no or minimum extra column band broadening effects so that the total column resolving power is maintained. The former objective is easily achieved by the on-column injection technique because non-vaporizing ("cold") on-column injection, unlike conventional vaporizing injection techniques (split, splitless or direct), can deliver a sample into a capillary gas chromatography column with little effect on composition. The discriminative, adsorptive and thermal effects commonly observed with vaporizing injectors are largely absent, and excellent quantitative accuracy and precision are obtainable. Thus, on-column injection has been successfully applied to a number of difficult sampling problems. As to the latter of the aforementioned objectives, however, intolerable band boardening has been produced by the injection of a liquid sample into a capillary column due to the dynamic spreading of the liquid sample by the carrier gas over a significant length of the column inlet. As described by K. Grob, Jr. in J. Chromatogr., Vol. 213 (1981) at page 3, an on-column injection of a large sample size can result in chromatographic peak splitting due to the effect of the column being flooded by the liquid sample. This liquid sample flooding not only reduces the total available column resolving power and lifetime but also provides minimal use for qualitative and quantitative chromatographic information. The extent of this flooding along the length of the column depends upon the sample size, the column diameter, the carrier gas flow rate, the solvent physicochemical properties, and the column temperature (which affects the viscosity of the carrier gas and surface tension of the liquid sample). In general, a sample size in the range of 1-2 microliters can typically flood a column length of more than 50 cm. A larger sample size up to 10 microliters can easily flood several meters of the column inlet. Thus, this initial spreading of the liquid sample zone is one of the most serious constraints on the use of the method, resulting not only in a non-reproducible peak profile depending on the distribution of the solute molecules within the flooded sample zone but also an extensive peak broadening which is determined by the initial sample bandwidth.
One of the attempts to reduce the effect of liquid sample flooding described by K. Grob, Jr. et al in J. Chromatogr., Vol. 244 (1982) at page 185 has been by removing the stationary phase on the first few meters of the column to prevent retention trapping of the non-uniformly distributed solute molecules. After injection, the flooded column inlet zone is heated up to vaporize sample molecules to be carried downstream to the column zone where stationary liquid traps solute molecules in a narrow initial sample zone. The technique improves the peak shape over that obtained with a conventional on-column injector. This technique, however, has limited success in practical applications due to the following drawbacks. Firstly, it is difficult in practice to strip stationary phase from a column inlet. In particular, nonpolar phases and chemically bonded phases are not completely removable. The use of an uncoated precolumn may allow satisfactory surface characteristics for the requirement of utilizing the retention gap technique, but the practical difficulties and constraints in column connection techniques have to be taken into consideration. Secondly, the retention gap technique does not solve the fundamental problem of sample size limitation. The amount of sample injected is again limited by the length of the retention gap. A sample size of 3 microliters may require 2-3 meters of retention gap to allow satisfactory peak shape. Thirdly, uncoated bare column walls for the retention gap may produce undesirable adsorption effects. Deactivation of the precolumn may not give satisfactory results due to the possibility of retention of solute molecules on the deactivated phase or phases, defeating the retention gap effect. Fourthly, the technique requires that the column oven temperature be cooled down to below the solvent boiling point before every injection. This could require more time than that required for a chromatographic separation. The speed of analysis is thus constrained by the injection technique.
It is therefore an object of this invention to provide a solute focusing method of introducing a liquid sample into a gas chromatographic column.
It is another object of this invention to provide an on-column injection method in gas chromatography which can yield chromatograms of good quality with relatively large sample sizes without causing intolerable peak shape distortion and, hence, useless chromatographic information.